1. Field of the Invention
The present invention relates to improvement of a diversity receiver in a field of radio communication.
2. Description of the Prior Art
A conventional diversity receiver is disclosed in, for example, JP-A-6/268559. The prior art will be described hereinafter with reference to the drawings. In the following discussion, a prior art structure is equivalent to, but not completely identical with, a structure disclosed in the JP-A-6/268559.
FIG. 22 is a block diagram showing a structure of the conventional diversity receiver. In the drawing, reference numeral 100 denotes a local oscillator, 810A, 810B, . . . , 810L are first, second to Lth (L: integer of two or more) differential detection/signal strength detecting circuits, 820A, 820B, . . . , 820L are first, second to Lth likelihood calculating circuits, 830A, 830B, . . . , 830L are first, second to Lth multipliers, 840 is a combining circuit, and 850 is a decision circuit.
A description will now be given of the operation. In FIG. 22, first, second to Lth (L: integer of two or more) received signals are respectively inputted into the first, second to Lth differential detection/signal strength detecting circuits 810A, 810B, . . . , 810L. On the other hand, the local oscillator 100 outputs a local carrier which is inputted into the first, second to Lth differential detection/signal strength detecting circuits 810A, 810B, . . . , 810L.
Here, the first, second to Lth received signals mean the same transmitted signal received by L antennas (not shown), which is modulated through differential M-phase (M: integer of two or more) phase shift-keying (hereinafter referred to as PSK) or differential .pi./M shift M-phase PSK. When a carrier frequency of the transmitted signal is defined as f, and a symbol duration thereof is defined as T, a value s.sub.k (iT) of the kth (k=1, 2, . . . , L) received signal at a time of t=iT (i: integer of zero or more) can be given by the following expression: EQU s.sub.k (iT)=.gamma..sub.k,i cos (2.pi.fiT+.psi..sub.k,i) (1)
If there are no effects due to noise, fading, and so forth, a value .psi..sub.k,i of a phase of the Kth received signal at the time of t=iT can be written as the following expression (where addition being made modulo 2.pi.) using an initial phase .theta..sub.o of the transmitted signal and a transmitted differential phase .DELTA..theta..sub.i determined by transmitted data: ##EQU1##
where the local carrier outputted from the local oscillator 100 has the same frequency as the carrier frequency f of the transmitted signal, and an initial phase thereof is .phi.. Therefore, when a value of the local carrier at the time of t=iT is defined as c(iT), the following expression can be held: EQU c(iT)=cos (2.pi.fiT+.phi.) (3)
The first, second to Lth differential detection/signal strength detecting circuits 810A, 810B, . . . , 810L respectively have the same structure, and perform the same signal processing of the first, second to Lth received signals. Thus, descriptions will be given of only the structure and the operation of the first differential detection/signal strength detecting circuit 810A.
FIG. 23 is a block diagram showing the structure of the first differential detection/signal strength detecting circuit 810A. In the drawing, reference numeral 210 denotes a phase comparator, 220 is a delay element with a delay time equal to a one-symbol duration T of the received signal, 230 is a subtracter modulo 2.pi., and 260 is a strength detecting circuit.
In FIG. 23, the first received signal, and the local carrier outputted from the local oscillator 100 are respectively inputted into the phase comparator 210. The phase comparator 210 outputs as a received phase signal a value of a phase of the first received signal on the basis of the local carrier. Thus, at the time of t=iT, a value of the received phase signal becomes .psi..sub.1,i -.phi. (subtraction being made modulo 2.pi.). The received phase signal is inputted into the delay element 220 with the delay time equal to the one-symbol duration T of the received signal, and the subtracter 230 modulo 2.pi.. The delay element 220 outputs a phase signal delayed by the one-symbol duration. Therefore, at the time of t=iT, a value of the phase signal delayed by the one-symbol duration becomes .psi..sub.1,i-1 -.phi.. The phase signal delayed by the one-symbol duration is inputted into the subtracter 230 modulo 2.pi.. The subtracter 230 subtracts modulo 2.pi. the phase signal outputted from the delay element 220 and delayed by the one-symbol duration from the received phase signal outputted from the phase comparator 210 to output the result of subtraction as a first one-symbol differential detection signal. Hence, when a value of the first one-symbol differential detection signal at the time of t=iT is defined as .DELTA..psi..sub.1,i, the following expression can be held (where subtraction being made modulo 2.pi.): EQU .DELTA..psi..sub.1,i =(.psi..sub.1,i -.phi.)-(.psi..sub.1,i-1 -.phi.)=.psi..sub.1,i -.psi..sub.1,i-1 ( 4)
That is, the first one-symbol differential detection signal .DELTA..psi..sub.1,i expresses a variation in phase for the one-symbol duration of the first received signal, and has a value equal to the transmitted differential phase .DELTA..theta..sub.i without the effects due to noise, fading, and so forth. As set forth above, since the value of the transmitted differential phase .DELTA..theta..sub.i is determined by the transmitted data, it is possible to estimate the transmitted data by using the value of the first one-symbol differential detection signal .DELTA..psi..sub.1,i.
Further, the first received signal is inputted into the strength detecting circuit 260. The strength detecting circuit 260 outputs as first received signal strength the square of amplitude of the first received signal. That is, the first received signal strength is proportional to signal power of the first received signal. If a value of the first received signal strength at the time of t=iT is defined as P.sub.1,i, the following expression can be held: EQU P.sub.1,i =.gamma..sub.1,i.sup.2 ( 5)
The above signal processing generates the first one-symbol differential detection signal and the first received signal strength which are outputted from the first differential detection/signal strength detecting circuit 810A. Returning to FIG. 22, a description will be given of the prior art.
According to the same signal processing as that in the first differential detection/signal strength detecting circuit 810A, the second to Lth differential detection/signal strength detecting circuits 810B, . . . , 810L generate and output the second to Lth one-symbol differential detection signals and received signal strength from the second to Lth received signals and the local carrier outputted from the local oscillator 100. Thus, when values of the kth (k=2, . . . , L) one-symbol differential detection signal and received signal strength at the time of t=iT are defined as .DELTA..psi..sub.k,i and P.sub.k,i, the following expressions can be held (where subtraction being made modulo 2.pi.): EQU .DELTA..psi..sub.k,i =.psi..sub.k,i -.psi..sub.k,i-1 P.sub.k,i =.gamma..sub.k,i.sup.2 ( 6)
The first, second to Lth differential detection/signal strength detecting circuits 810A, 810B, . . . , 810L output the first, second to Lth one-symbol differential detection signals which are respectively inputted into the first, second to Lth likelihood calculating circuits 820A, 820B, . . . , 820L. The first, second to Lth likelihood calculating circuits 820A, 820B, . . . , 820L are operated for the same signal processing of the first, second to Lth one-symbol differential detection signals. Hence, a description will now be given of only the operation of the first likelihood calculating circuit 820A.
As set forth above, the first one-symbol differential detection signal .DELTA..psi..sub.1,i is equal to the transmitted differential phase .DELTA..theta..sub.i without the effects due to noise, fading, and so forth. Further, the transmitted differential phase .DELTA..theta..sub.i takes any one value of M signal point phases .alpha..sub.0, .alpha..sub.1, . . . , .alpha..sub.M-1 according to a value of the transmitted data. Therefore, it is possible to regard an absolute value .vertline..DELTA..psi..sub.1,i -.alpha..sub.m .vertline. of a difference between the first one-symbol differential detection signal .DELTA..psi..sub.1,i and each of the signal point phases .alpha..sub.m (m=0, 1, . . . , M-1) as the likelihood showing probability that the value of the transmitted differential phase .DELTA..theta..sub.i is .alpha..sub.m. Here, it is assumed that the subtraction is made modulo 2.pi., and the result of subtraction is equal to or more than -.pi. and less than .pi.. In this case, it is shown that the probability becomes higher as the value of the likelihood becomes smaller. Further, a value of each of the signal point phases .alpha..sub.m is defined as .alpha..sub.m =2 m.pi./M in case of the differential M-phase PSK, or .alpha..sub.m =2 m.pi./M+.pi./M in case of the differential .pi./M shift M-phase PSK.
The first likelihood calculating circuit 820A calculates for combination of all likelihood .lambda..sub.1,i,m =.vertline..DELTA..psi..sub.1,i -.alpha..sub.m .vertline. of the first one-symbol differential detection signal with respect to the respective signal point phases .alpha..sub.m (m=0, 1, . . . , M-1), and outputs the result as a first likelihood signal .lambda..sub.1,i =(.lambda..sub.1,i,0, .lambda..sub.1,i,1, . . . , .lambda..sub.1,i,M-1). In the calculation of the likelihood .lambda..sub.1,i,m, it is assumed that the subtraction is made modulo 2.pi., and the result of subtraction is equal to or more than -.pi. and less than .pi..
The second to Lth likelihood calculating circuits 820B, . . . , 820L carry out the same signal processing as that in the first likelihood calculating circuit 820A, and calculate and output second to Lth likelihood signals from the second to Lth one-symbol differential detection signals. Therefore, for likelihood .lambda..sub.k,i,m (m=0, 1, . . . , M-1) constituting a kth (k=2, . . . , L) likelihood signal .lambda..sub.k,i =(.lambda..sub.k,i,0, .lambda..sub.k,i,1, . . . , .lambda..sub.k,i,M-1 ), the following expression can be held (where the subtraction is made modulo 2.pi., and the result of subtraction is equal to or more than -.pi. and less than .pi.): EQU .lambda..sub.k,i,m =.vertline..DELTA..psi..sub.k,i -.alpha..sub.m .vertline.(7)
The first, second to Lth likelihood calculating circuits 820A, 820B, . . . , 820L output the first, second to Lth likelihood signals .lambda..sub.1,i, .lambda..sub.2,1, . . . , .lambda..sub.L,i which are respectively inputted into the first, second to Lth multipliers 830A, 830B, . . . , 830L. Further, the first, second to Lth differential detection/signal strength detecting circuits 810A, 810B, . . . , 810L output the first, second to Lth received signal strength P.sub.1,i, P.sub.2,i, . . . , P.sub.L,i which are similarly inputted into the first, second to Lth multipliers 830A, 830B, . . . , 830L to be multiplied by the first, second to Lth likelihood signals .lambda..sub.1,i, .lambda..sub.2,1, . . . , .lambda..sub.L,i. The results of multiplication are inputted into the combining circuit 840 which adds up the results to provide and output a combined likelihood signal .LAMBDA..sub.i =(.GAMMA..sub.i,0, .GAMMA..sub.i,1, . . . , .GAMMA..sub.i,M-1) Hence, combined likelihood .LAMBDA..sub.i,m (m=0, 1, . . . , M-1) serving as components of the combined likelihood signal .LAMBDA..sub.i can be given by the following expression: ##EQU2##
That is, the combined likelihood .LAMBDA..sub.i,m (m=0, 1, . . . , M-1) can be obtained through diversity combining after weighting the likelihood .lambda..sub.k,i,m (k=1, 2, . . . , L) with respect to the signal point phases .alpha..sub.m with the received signal strength P.sub.k,i.
The combining circuit 840 outputs the combined likelihood signal .LAMBDA..sub.i which is inputted into the decision circuit 850. As set forth above, it is shown that the probability becomes higher as the value of the likelihood becomes smaller. Consequently, in the decision circuit 850, it is determined that the value of the transmitted differential phase .DELTA..theta..sub.i is equal to a signal point phase .alpha..sub..mu. with respect to the minimum value .LAMBDA..sub.i,.mu. (.mu..epsilon.{0, 1, . . . , M-1}) among the combined likelihood .LAMBDA..sub.i,m (m=0, 1, . . . , M-1) serving as components of the combined likelihood signals .LAMBDA..sub.i. Subsequently, the decision circuit 850 outputs data corresponding to the signal point phase .alpha..sub..mu. as demodulated data depending upon a correspondence between the transmitted data and the transmitted differential phase .DELTA..theta..sub.i.
In such a manner, the conventional diversity receiver can provide a diversity effect by determining the demodulated data by using the combined likelihood .LAMBDA..sub.i,m obtained through the diversity combining after weighting the likelihood .lambda..sub.k,i,m (k=1, 2, . . . , L) with respect to the signal point phases .alpha..sub.m (m=0, 1, . . . , M-1) with the received signal strength P.sub.k,i.
As set forth above, in the conventional diversity receiver, the demodulated data is determined on the basis of the likelihood signal generated from the one-symbol differential detection signal. As described above, the one-symbol differential detection signal is obtained by subtracting a received signal phase preceding by the one-symbol duration from a current received signal phase. However, both the received signal phases are generally affected by independent noises. Thus, the one-symbol differential detection signal has a lower signal-to-noise power ratio (hereinafter briefly referred to as SNR) than that of the received signal. In the conventional diversity receiver, the demodulated data is determined on the basis of the likelihood signal generated from the one-symbol differential detection signal having the lower SNR than that of the received signal. As a result, there is a problem of an inferior bit error rate performance of the demodulated data.